Conversion of depleted uranium hexafluoride to a solid uranium compound

ABSTRACT

A process for converting UF 6  to a solid uranium compound such as UO 2  and CaF. The UF 6  vapor form is contacted with an aqueous solution of NH 4 OH at a pH greater than 7 to precipitate at least some solid uranium values as a solid leaving an aqueous solution containing NH 4 OH and NH 4 F and remaining uranium values. The solid uranium values are separated from the aqueous solution of NH 4 OH and NH 4 F and remaining uranium values which is then diluted with additional water precipitating more uranium values as a solid leaving trace quantities of uranium in a dilute aqueous solution. The dilute aqueous solution is contacted with an ion-exchange resin to remove substantially all the uranium values from the dilute aqueous solution. The dilute solution being contacted with Ca(OH) 2  to precipitate CaF 2  leaving dilute NH 4 OH.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. DE-AC02-98CH10913 between the U.S. Department of Energy(DOE) and The University of Chicago representing Argonne NationalLaboratory.

BACKGROUND OF THE INVENTION

There are large worldwide stored inventories of the depleted uraniumhexafluoride (UF₆) tailings from past gaseous-diffusion-enrichmentplants for nuclear fuel-cycle operations. In the U.S. alone, there arecurrently ≈700,000 metric tons of that material stored in U.S DOEfacilities at Paducah, Ky., Portsmouth, Ohio and Oak Ridge, Tenn. Oneconversion process considered in the past by the DOE would use steam forhigh-temperature hydrolysis of the hexafluoride, to convert it to (a)UO₂ for use in other nuclear programs or for disposal, and (b) anhydrousHF for industrial use. The present invention relates to alow-temperature aqueous process to convert the hexafluoride to UO₂ foruse in other nuclear programs, and high-purity calcium fluoride forsale.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forconverting UF₆ to a solid uranium compound, comprising: contacting UF₆vapor with an aqueous solution of NH₄OH at a pH greater than 7 toprecipitate at least some solid uranium values as a solid leaving anaqueous solution containing NH₄OH and NH₄F and remaining uranium values,separating the solid uranium values from the aqueous solution of NH₄OHand NH₄F and remaining uranium values, diluting the aqueous solution ofNH₄OH and NH₄F and remaining uranium values with additional waterprecipitating more uranium values as a solid, leaving trace quantitiesof uranium in a dilute aqueous solution.

Another object of the present invention is to provide a process forconverting UF₆ to a solid uranium compound and CaF₂, comprising:contacting UF₆ vapor with an aqueous solution of NH₄OH at a pH greaterthan 7 to precipitate at least some solid uranium values as a solidleaving an aqueous solution containing NH₄OH and NH₄F and remaininguranium values, separating the solid uranium values from the aqueoussolution of NH₄OH and NH₄F and remaining uranium values, diluting theaqueous solution of NH₄OH and NH₄F and remaining uranium values withadditional water precipitating more uranium values as a solid leavingtrace quantities of uranium in a dilute aqueous solution, contacting thedilute aqueous solution with an ion-exchange resin to removesubstantially all the uranium values from the dilute aqueous solution,and contacting the dilute aqueous solution having substantially alluranium values removed with Ca(OH)₂ to precipitate CaF₂ leaving diluteNH₄OH.

The invention consists of certain novel features and a combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects andadvantages may best be understood from the following detaileddescription of the embodiment of the invention illustrated in thedrawings, wherein:

FIG. 1 is a flow chart illustrating the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The uranium as UO₂(OH)₂ is precipitated at a higher pH such as in therange of 7 to about 12 by adding UF₆ to the ammonium hydroxide “dropvessel”. The trace uranium impurities are extracted from the supernatantfluoride solution before treatment of the supernatant solution withcalcium hydroxide to produce calcium fluoride. The advantage of thisapproach is that it would not require prohibitively large quantities ofan ion exchange resin, such as Diphonix®, see Horwitz et al. U.S. Pat.Nos. 5,539,003, 5,449,462, 5,332,531 and 5,281,631, the disclosures ofwhich are incorporated by reference, for those extractions. In addition,the high capacity of the Diphonix® resin for binding the uranium makesit economically attractive to discard it after it has reached its limitof uranium content, rather than trying to regenerate it by strippingthat uranium with ammonium carbonate to generate uranyl carbonate. Thelatter approach, however, could be used if disposition of the Diphonix®waste stream presents a problem. In this approach, the uranyl carbonateis treated with ammonium hydroxide to precipitate additional uranylhydroxide and regenerate the ammonium carbonate reagent. It should beunderstood that a variety of ion exchange resins may be used in theinventive process, but the Diphonix® resin is disclosed by way ofexample only, not by way of limitation.

In the first step of the aqueous conversion process, depleted UF₆ isvaporized by sublimation at 56° C. or above and its condensate deliveredto a “drop vessel” containing moderately concentrated ammonium hydroxidehaving a pH in the range of from about 7 to about 12.

The course of the reaction is followed by repeated pH measurements andvisual observation of any precipitates formed, as well as chemicalanalyses of the solid and aqueous phases. Although the reactions arecomplex in the presence of the concentrated fluorides formed fromadditions of UF₆ to concentrated NH₄OH, the stoichiometry for the courseof the reactions, past the end point for the precipitation of UO₂(OH)₂,may be given as

2UF₆+14NH₄OH(aq)=(NH₄)₂U₂O₇(c)+12NH₄F(aq)+7H₂O  (1)

2UF₆+12NH₄OH(aq)=2UO₂OH₂(c)+12NH₄F(aq)+4H₂O  (2)

2UF₆+10NH₄OH(aq)=2UO₂(OH)F(aq)+10NH₄F(aq)+4H₂O  (3)

(NH₄)₂U₂O₇ might not be precipitated with additional of UF₆ to diluteNH₄OH, if that diuranate could be hydrolyzed in the back-reaction forthe postulated equilibrium equation

2NH₄OH(aq)+2UO₂(c)=(NH₄)₂U₂O₇(c)+3H₂O  (4)

Experiments were conducted to represent key points for addition of UF₆to moderately concentrated NH₄OH in the stoichiometry ranges betweenEqs. 1 and 3. For that purpose, a mixture was made up to the approximateas-added concentrations of

0.5 M UO₂F₂, 1.0 M NH₄F and 0.6 M NH₄OH.

That mixture represents the addition of U(VI) to NH₄OH to reach ahydroxide-to-UO₂++ ion ratio between those at the end-points of Eq's. 2and 3, but with a fluoride-to-uranium ratio of 4, rather than the ratioof 6 that would result from the addition of UF₆ to NH₄OH. The purpose ofthe initial preparation was to add additional NH₄OH to it, as required,to ensure effective precipitation of uranium, analyze the supernatantliquid, and dilute it (if required) to achieve uranium impurityconcentrations suitable for extraction with Diphonix® ion exchangeresin.

UO₂F₂ was prepared by adding water and ≈5 mL of 48%, analytical-grade HFto a Teflon® beaker containing 7.1516 g of UO₃, 25 mmol (millimoles),and gently heating the mixture to prevent spattering, to first effectdissolution. The excess HF was then removed by evaporation to dryness.From the factor weights of UO₃ and UO₂F₂, the dried solid contained7.7016 g of UO₂F₂. However, due to the hygroscopic nature of UO₂F₂, theas-dried solid contained 11.854 g of water at the time it was weighed.25.797 g of additional water were added to the deliquescent solid, andthe mixture was warmed to form a canary-yellow UO₂F₂ solution designatedas Solution A.

Solution B was produced by adding 23.315 g of water to another 100-mLbeaker containing 1.829 g of NH₄F and 1.916 g of an aqueous solution of28% analytical grade NH₃ in water. From the factor weight, therefore,Solution B contained 49.38 mmol of NH₄F. At 28% NH₃, each gram of the14.8 M NH₃ reagent contained 16.43 mmol of either NH₃, or NH₄OH. SoSolution B contained 31.48 mmol of NH₄OH. In summary, the combinedcontents of Solutions A and B were

25.00 mmol of UO₂F₂,

49.38 mmol of NH₄F, and

31.48 mmol of NH₄OH.

Solution A was added slowly to Solution B with stirring to form SolutionC. No precipitation was formed in Solution C because the ratio of addedhydroxide to that of UO₂ ²⁺ ion was only 26% greater than that for asolution of the highly soluble salt UO₂(OH)₂.UO₂F₂.H₂O. Therefore,several more milliliters of the concentrated NH₄OH had to be addedbefore observable precipitation of uranyl hydroxide. This furtheraddition of 14.8 M NH₄ OH was followed by checking the aqueous phasewith pH paper. The quantity of additional NH₄OH that was added was notmeasured. It was estimated that the total addition of NH₄OH was 3-5times the 31.48 mmol amount originally present in Solution C.

The mixture of aqueous and solid phases from addition of excess NH₄OH toSolution C was divided between two 50-mL polypropylene centrifuge tubes.The solid settled nicely after ≈2 minutes, but the canary-yellowappearance of the supernatant liquid indicated the presence of dissolveduranyl products still remaining in the aqueous phase. The supernatantfrom the two centrifuge tubes were combined, and the pH of the ≈60-mL ofthat supernatant, Solution S. was 8.92. A 500 μL aliquot of thesupernatant, Solution S, was diluted to 10 mL with water for a 1/20dilution and submitted to a laboratory for uranium analysis. From thatanalysis, it was determined that the as-diluted uranium content of thealiquot sample was 257 μg/mL, or a total uranium content of 5.14 mg/mL,i.e., 0.0216 M total uranium in S. The analytical chemistrydeterminations for the concentrations of constituents for ≈ 60 mL ofSolution S were

[U]_(T)=0.0216 M,

[F]_(T)=0.656 M,

and

a_(OH)−=8.32×10⁻⁶ M,

where [U]_(T) is the total concentration of uranium, in the uranylfluoride complexes; and [F]_(T) is the total concentration of complexedand noncomplexed gram atoms of fluoride per mL. a_(OH−), the hydroxylion activity, was determined from the pH measurement of 8.92 and therelations

pH=loga_(H+),

pOH+loga_(OH−),

and

pH+pOH=14,

where a_(H+) represents the hydrogen ion activity.

It should be understood that while ion activities are their activitycoefficients are frequently expressed in terms of molality, i.e., molesper 1000 g of solvent, the concentrations given here have all been interms of molarity, i.e., moles per liter of solution. Those solutionswere derived, from dividing the millimolar quantities of additives inthe initial preparation by the approximate 60-mL volume of thatpreparation. But there is no appreciable difference between thepolarities and molalities of those constituents within the uncertaintyof that volume. That is so because the total mass of the water solventin S. i.e., the sum of the masses of water in A, B, the as-dried UO₂F₂,and the 1.916 g of 28% NH₃ reagent was determined to be 62.341 g.

The 0.0216 M concentration of uranium in S was too large to be processedfor uranium extraction without using a prohibitively large amount of theDiphonix® ion-exchange resin. Therefore, samples were prepared for suchtests from 1/10, 1/20, and 1/50 dilutions of S. Furthermore, due toequilibria that will be discussed later, it was found that thosedilutions, as with the 1/20 diluted aliquot taken for the first ICPanalysis, resulted in the precipitation of additional uranyl materialafter the diluted material was allowed to stand.

The effect of additional precipitation on dilution leads to a betterextraction plan for the overall conversion process, and is an importantpart of the invention. As shown in FIG. 1, the supernatant from thefirst precipitation of uranyl material was diluted to effect additionalprecipitation by the dilution. Then the supernatant from that dilutionwas processed to extract its trace uranium contents with the Diphonix®ion-exchange resin. Subsequently, the highly purified diluted solutionwas treated with CA(OH)₂ to precipitate CaF₂. An option for the recoveryof the NH₃ reagent is to distill it from the supernatant from thatprocess step and collect it in water to concentrate it.

The pH values of the supernatant from the 1/10, 1/20, and 1/50 dilutionswere measured with a pH meter. An aliquot of the liquid phase resultingfrom each dilution, after it settled, was submitted to the laboratoryand its [U]_(T) concentration was measured in a preliminary ICPanalysis. Table I gives the characterizations of the supernatant S andits dilutions, designated in the table as D(1/10), D(1/20), and D(1/50).

The pH values were used to determine the respective OH-ion activities.

With the concentrations of constituents in Solution C, beforeprecipitation of uranium, the possible soluble fluoride complexes areUO₂F, UO₂F₂, UO₂F₃ ⁻, UO₂F₄ ²⁻, and UO₂F₅ ³⁻. Formation of thosecomplexes with UO₂ ²⁺ competes with the reaction between UO₂ ²⁺ and theOH-(from NH₄OH) to form UO₂(OH)₂. Consequently, it is assumed that whenthe stoichiometric quantity of NH₄OH necessary to precipitate UO₂(OH)₂is added to aqueous UO₂F₂, a small amount of the uranium that would haveprecipitated is tied up as soluble fluoride complexes. Therefore, asobserved in the addition of extra NH₄OH to the target Solution C,precipitation did not occur until a considerable excess of NH₄OH wasadded, with a residual 0.0216 M concentration of total uranium remainingin the supernatant. Assuming that UO₂(OH)₂ was the only compoundprecipitated from solution, the equilibrium phenomena involved withthose effects might be represented by some combination of equations suchas

UO₂(OH)₂(c)=UO₂ ²⁺(aq)+2OH⁻(aq)  (5)

UO₂(OH)₂(c)+F⁻(aq)=UO₂F⁺(aq)+2OH⁻(aq)  (6)

UO₂(OH)₂(c)+2F⁻(aq)=UO₂F₂(aq)+2OH⁻(aq)  (7)

UO₂(OH)₂(c)+3F⁻(aq)=UO₂F₃ ⁻(aq)+2OH⁻(aq)  (8)

UO₂(OH)₂(c)+4F⁻(aq)=UO₂F₄ ²⁻(aq)+2OH⁻(aq)  (9)

UO₂(OH)₂(c)+5F⁻(aq)=UO₂F₅ ³⁻(aq)+2OH⁻(aq)  (10)

The OH−ion released from the forward reactions is mostly tied up byreaction with the NH₄ ⁺ ion to buffer the solution somewhat through theequilibrium

NH₄ ⁺(aq)+OH⁻(aq)=NH₄OH(aq)  (11)

where the dissociation constant for NH₄OH is given by $\begin{matrix}{K_{11} = {\frac{{{}_{}^{}{}_{}^{}} + {{}_{}^{}{}_{}^{}}}{{{}_{}^{}{}_{}^{}}{OH}} = {1.82 \times 10^{- 5}}}} & (12)\end{matrix}$

and where the a values represent the thermodynamic activities of thechemical entities in Eq. 12. Clarification of that matter is given belowin a discussion of the dissociation constant of NH₄OH.

Because of the buffering effect quantified in Eq. 12, dilution of thesupernatant, Solution S, would cause significantly more reduction of theF− ion concentration than that of the OH− ion concentration. Therefore,it would be expected that equilibrium Eqs. 6-10 would be shifted more tothe left as the fluoride concentration is reduced by dilution of thesupernatant. That expectation is in agreement with the experimentalobservations that more UO₂(OH)₂ precipitated with dilution of thesupernatant. Furthermore, the NH₄OH dissociated to supply more OH− ionsfor that precipitation with a consequent increase of the$\frac{{}_{}^{}{}_{}^{}}{{{}_{}^{}{}_{}^{}}{OH}}$

ratio of the supernatant. That effect together with Eq. 12 would explainthe gradual, i.e., somewhat buffered, trend toward lower pH-values (inaddition to the precipitation of UO₂(OH)₂) that was observed withdilution of the supernatant. Also, it is possible that some of the NH₄OHreacted with the UO₂(OH)₂ precipitate to produce ammonium diuranate.Consequently, it was not possible to determine the total amount of NH₄OHthat was added to precipitate U(VI) from that solution.

In a consideration of the ionic strength of Solution S and its effect onactivity coefficients, it was assumed that the fluoride ionconcentration was ≈1.66 M, even though it was somewhat lower because ofthe fluoride tied up as uranyl fluoride complexes in the 0.0216 Mconcentration of total uranium. From the electrical neutrality of thesolution, the NH₄ ⁺ ion concentration would also be ≈1.66 M. Publishedvalues of mean activity coefficients for a number of electrolytes over amolality range of 0.001 to 3.0 are available. No data are given forNH₄F, but it is assumed to have the same mean activity coefficient asNaCl which has the same mean ionic diameter as NH₄F. At a molality,molarity, or ionic strength of 1.66, the mean activity coefficient, y±of NaCl is ≈0.66. Therefore, because individual activity coefficientscannot be measured directly, t is assumed that the activity coefficientsof both NH₄ ⁺ and F− ions were ≈0.66 and that $\frac{K_{11}}{k_{11}}$

The application of the Diphonix® ion-exchange resin in the extraction oftransuranic and other metals from aqueous media has been described. Suchapplications have found that the resin's performance for extraction ofU(IV) and U(VI) is generally improved with more acidic aqueous media.That would be of particular benefit for the extraction of U(VI) fromaqueous fluoride media, in which the fluoride could be tied up as eitherHF₂ ⁻ or HF rather than as fluoride complexes of the uranyl ion. But theperformance of Diphonix® ion-exchange resin in extracting U(VI) fromweakly basic solutions of NH₄OH and NH₄F was not known. Therefore, theprocedure of removing U(VI) impurities from weakly basic NH₄OH—NH₄Fmedia is new.

Accordingly, 4-mL aliquot samples of the diluted supernatant Solution Swere taken, after settling of additional uranium precipitates thatresulted from those dilutions, for interaction with the Diphonix®ion-exchange resin, along with sibling 4-mL control samples that werenot subjected to such interactions. The diluted controls samples aredesignated as DU(1/10), DU(1/20), and DU(1/50). Their total uraniumconcentrations, U_(o), to indicate the noninteracted concentrations,were measured at a laboratory as 37.4, 15.5, and respectively. Thesealiquots were allowed to stand longer than the D(1/10), D(1/20), andD(1/50) aliquots taken previously for the preliminary (total) uraniumanalyses, which for comparison were 64.5, 5.00, and 3.09 μg/mL,respectively. It is believed that the DU sample values are morereliable, due to their longer setting time.

The wet Diphonix® ion-exchange resin used for the study was a mixture of200-mesh solid Diphonix® particles and water. A sample of the wetDiphonix® ion-exchange resin that was used for the example was weighed,dried, and reweighed to determine that the dry (or active) part of themixture was 29.82%. Then, 4-mL aliquots of each of the supernatantliquids from those dilutions, designated DD(1/10), DD(1/20), andDd(1/50), were each mixed with about 0.22 to 0.24 g of the wet resin,containing 66 to 72 mg, respectively, of the dry (or active) Diphonix®ion-exchange resin component. An equilibrium distribution of uraniumbetween the liquid and solid (Diphonix®) ion-exchange resin phase iseach of the batch mixtures was then produced by using a magnetic stirrerfor ≈1.5 hr. After establishing equilibrium, a liquid-phase sample wasremoved from each of the batch mixtures for analysis, using a f-mLsyringe with a 0.8-μm polycarbonate filter.

The dry weight distribution factor, D, was determined for each of thethree extractions from the relation$D = \frac{\left\{ \frac{U_{o} - U_{f}}{W} \right\}}{\left\{ \frac{U_{f}}{V} \right\}}$

where

U_(o) is the uranium concentration in a solution that was not interactedwith Diphonix® ion-exchange resin,

U_(f) is the uranium concentration in that solution after itsinteraction with Diphonix® ion-exchange resin,

W is the dry weight of Diphonix® ion-exchange resin for that interactionin grams, and

V is the volume of interacted sample in mL.

The results of those experiments are given in Table 2. In all cases, Vand 4 mL.

It was determined that by the combined effects of either a 1-to 20 or1-to-50 dilution (with the phenomenon of additional removal of uraniumby precipitation in those dilutions) and ion-exchange with the Diphonix®resin, the inventive process produced a uranium content in theNH₄F—NH₄OH solution of 30-40 parts per billion (ppb), starting with theoriginal content of 5.14 parts per thousand in supernatant Solution S.As indicated in FIG. 1, Ca(OH)₂ is added to the highly purifiedNH₄F—NH₄OH solution to yield CaF₂, rather than using the reverse orderof addition. With that order of addition, the NH₄ ⁺ of the NH₄F—NH₄OHcouple in the purified solution scavenges OH− ion from the Ca(OH)₂ toreduce the OH− ion concentration to a value many orders of magnitudelower than that from the saturated solubility of pure Ca(OH)₂.Consequently, the lower pH values rom the preferred order of addition,i.e., the addition of Ca(OH)₂ to the NH₄F—NH₄OH rather than the reverseorder of addition, should cause less coprecipitation of uraniumimpurities to yield a very pure CaF₂ product.

The D values reflect the distribution of positive ions like UO₂ ²⁺ andUO₂F⁺ (that can be bound to the Diphonix®) ion-exchange resin betweenthe Diphonix® ion-exchange resin and the aqueous phase. Therefore, theexperimental results indicate there was a greater proportion of thosepositive ions in the 1-to-20 diluted solution than in the 1-to-10diluted solution. However, it also appears that there might have beensome base level (≈30 parts per billion) of uranium-bearing material thatwas nonreactive with the Diphonix® ion-exchange resin.

Based on the experiments performed, it is believed that settling time iscritical to obtaining the best results and should occur before analyzingthe supernatant solution from a diluted control sample. Therefore, it isrecommended that sibling aliquots of those dilutions used for controlanalyses should not only be allowed to stand, but they should also bestirred with a magnetic stirrer, similar to that used for their siblingaliquots that were mixed and interacted with Diphonix® ion-exchangeresin for the extraction studies in these examples. It is believed to beadvisable to stir all parts of a diluted solution for betterequilibrium.

While there has been disclosed what is considered to be the preferredembodiment of the present intention, it is understood that variouschanges in the details may be made without departing from the spirit, orsacrificing any of the advantages of the present invention.

What is claimed is:
 1. A process for converting UF₆ to a solid uraniumcompound, comprising: contacting UF₆ with an aqueous solution of NH₄OHat a pH greater than 7 to precipitate at least some solid uranium valuesas a solid leaving an aqueous solution containing NH₄OH and NH₄F andremaining uranium values, separating the solid uranium values from theaqueous solution of NH₄OH and NH₄F and remaining uranium values,diluting the aqueous solution of NH₄OH and NH₄F and remaining uraniumvalues with additional water precipitating more uranium values as asolid leaving trace quantities of uranium in a dilute aqueous solution.2. The process of claim 1, wherein the process is conducted at a pH inthe range of greater than 7 to about
 12. 3. The process of claim 1,wherein solid UO₂(OH)₂ is produced and thereafter reduced to solid UO₂.4. The process of claim 1, wherein the aqueous solution of NH₄F andNH₄OH and remaining uranium values is diluted by adding water in excessof ten times the volume of the aqueous solution of NH₄F and NH₄OH andremaining uranium values.
 5. The process of claim 4, wherein the addedwater is in excess of between about twenty and about fifty times thevolume of the aqueous solution of NH₄F and NH₄OH and remaining uraniumvalues.
 6. The process of claim 1, and further including the step ofretaining the diluted aqueous solution of NH₄OH and NH₄F and remaininguranium values for a time sufficient to allow solid uranium values tosettle out leaving trace quantities of uranium.
 7. The process of claim1, wherein the uranium concentration after contacting the dilute aqueoussolution with an ion exchange resin is less than about 40 ppb and theion exchange resin is phosphoric acid based.
 8. A process for convertingUF₆ to a solid uranium compound and CaF₂, comprising: contacting UF₆with an aqueous solution of NH₄OH at a pH greater than 7 to precipitateat least some solid uranium values as a solid leaving an aqueoussolution containing NH₄OH and NH₄F and remaining uranium values,separating the solid uranium values from the aqueous solution of NH₄OHand NH₄F and remaining uranium values, diluting the aqueous solution ofNH₄OH and NH₄F and remaining uranium values with additional waterprecipitating more uranium values as a solid leaving trace quantities ofuranium in a dilute aqueous solution, contacting the dilute aqueoussolution with an ion-exchange resin to remove substantially all theuranium values from the dilute aqueous solution, and contacting thedilute aqueous solution having substantially all uranium values removedwith Ca(OH)₂ to precipitate CaF₂ leaving dilute NH₄OH.
 9. The process ofclaim 8, wherein the process is conducted at a pH in the range ofgreater than b 7 to about
 12. 10. The process of claim 8, wherein theUF₆ is added to NH₄OH having a pH in excess of about
 11. 11. The processof claim 8, wherein solid UO₂(OH)₂ is produced and thereafter reduced tosolid UO₂.
 12. The process of claim 11, wherein the solid UO₂(OH)₂ isreduced to UO₂ by contact with hydrogen gas.
 13. The process of claim 8,wherein the aqueous solution of NH₄F and NH₄OH and remaining uraniumvalues is diluted by adding water in excess of ten times the volume ofthe aqueous solution of NH₄F and NH₄OH and remaining uranium values. 14.The process of claim 13, wherein the added water is in excess of betweenabout twenty and about fifty times the volume of the aqueous solution ofNH₄F and NH₄OH and remaining uranium values.
 15. The process of claim 8,wherein the dilute aqueous solution is contacted with a disphosphoricacid based ion-exchange resin and the resultant solution has a uraniumconcentration of less than about 40 ppb.
 16. The process of claim 8, andfurther including the step of retaining the diluted aqueous solution ofNH₄OH and NH₄F and remaining uranium values for a time sufficient toallow solid uranium values to settle out leaving trace quantities ofuranium.
 17. The process of claim 16, wherein the diluted aqueoussolution of NH₄OH and HF and remaining uranium values are agitatedduring the retaining step.
 18. The process of claim 8, wherein theprocess is conducted continuously.
 19. The process of claim 8, whereinthe process is conducted in batches.